The present invention relates to a direction finder for use in a navigation system of a vehicle, which is capable of finding a moving direction of the vehicle on the basis of terrestrial magnetism.
There have been many navigation systems of this type and they are based on a principle which is shown in FIGS. 1 and 2. In FIG. 1, a horizontal component H of terrestrial magnetism, which is referred to as "terrestrial magnetism H" hereinafter, is detected by a terrestrial magnetism sensor 2 mounted on a vehicle 1, e.g., an automobile, whose heading makes an angle .theta. with respect to the direction of terrestrial magnetism H, i.e., north. That is, the sensor 2 detects a field component Hy (=H cos .theta.) of the magnetism H which is parallel to the moving direction A of the vehicle 1 and a field component Hx (=H sin .theta.) orthogonal to the direction A and provides electric signals x and y in the form of, for example, voltages corresponding thereto. The electric signals x and y are amplified suitably. Thus, the electric signals x and y can be expressed by EQU x=K Hx=K H sin .theta. (1a) EQU y=K Hy=K H cos .theta. (1b)
where K is a magnetism/voltage conversion coefficient.
The detected signals x and y when the field components Hx and Hy are zero are calibrated to zero so that the magnitudes of the signals x and y are proportional to the intensities of the components Hx and Hy, respectively and can be used as reference values.
FIG. 2 shows an x-y rectangular coordinate system on which points each defined by magnitudes of the electric signals x and y are plotted. A locus of the plot describes a circle O.sub.1 and the angle .theta., i.e., the orientation .theta.of the vehicle 1 is given by EQU .theta.=tan.sup.-1 (x/y) (2)
The direction of the terrestrial magnetism H is not coincident with the geographical north and there is an error, i.e., declination therebetween. The declination depends on the area of the earth in which it is calculated. It is assumed in this description, however, that there is no declination for simplicity of explanation.
It has been known that, due to magnetization of magnetic material of various components constituting the vehicle, the orientation .theta. calculated according to the equation (2) is not always correct.
Describing this in more detail with respect to FIGS. 3 and 4, the vehicle 1 is subjected to a magnetic field Hv shown in FIG. 3 produced by those magnetized components. With the magnetic field Hv, the magnetic field to be detected by the terrestrial magnetism sensor 2 becomes a composite magnetic field He of the terrestrial magnetism H and the magnetic field Hv. Coordinates (x,y), (x.sub.v,y.sub.v) and (x.sub.e,y.sub.e) of signals from the sensor 2, corresponding to coordinates (Hx,Hy), (Hvx,Hvy) and (Hex,Hey), are shown in FIG. 4. Thus, the signals xe and ye from the sensor 2 are represented by EQU x.sub.e =x+x.sub.v =KH sin .theta.+x.sub.v (3a) EQU y.sub.e =y+y.sub.v =KH cos .theta.+y.sub.v (3b)
where the angle .theta..sub.e obtained from the signals x.sub.e and y.sub.e according to the equation (2) becomes EQU .theta..sub.e =tan.sup.-1 (x.sub.e /y.sub.e) (4)
Thus, a true orientation .theta. cannot be obtained.
However, since the field Hv is produced by the vehicle 1 as a permanent magnet and an intensity and direction thereof with respect to the moving direction A of the vehicle 1 are constant, the coordinates (x.sub.v,y.sub.v) of the signal corresponding to the magnetic field Hv shown in FIG. 4 is kept unchanged even if the direction A is changed. Therefore, a locus of the coordinates (x.sub.e,y.sub.e) of the detection signal when the vehicle 1 runs once along a circle becomes a circle O.sub.2 having a center point (x.sub.v,y.sub.v) as is clear from the equations (3a) and (3b). Therefore, by obtaining the center coordinates (x.sub.v,y.sub.v) of the circle O.sub.2 from the detection signals x.sub.e and y.sub.e, the true orientation .theta. can be obtained easily from the following equation EQU .theta.=tan.sup.-1 ((x.sub.e -x.sub.v)/(y.sub.e -y.sub.v)) (5)
Japanese Patent Application Laid-Open No. 148210/1982 discloses a technique by which the true orientation .theta. is obtained by cancelling out influences of the magnetic field Hv on the basis of the principle mentioned above. In detail, among the detection signals x and y obtained from the terrestrial magnetism sensor 2 when the vehicle 1 circles once, maximum values x.sub.max and y.sub.max and minimum values x.sub.min and y.sub.min in the respective axes of the x-y rectangular coordinate system are stored and the detection signals x.sub.v and y.sub.v corresponding to the magnetic field Hv are obtained as coordinates of the center of the circular locus O.sub.2, according to the following equations EQU x.sub.v =(X.sub.max +x.sub.min)/2 (6a) EQU y.sub.v =(y.sub.max +y.sub.min)/2 (6b)
Therefore, by turning around the vehicle 1 in a suitable time to obtain the detection signals x.sub.v and y.sub.v corresponding to the magnetic field Hv, it is possible to obtain the true orientation .theta. by performing an operation of the equation (5).
However, when the vehicle 1 is, for example, an automobile, it is subjected to vibrations during its movement. Therefore, the magnetic field Hv may vary gradually as shown in FIG. 5, although the variation might be negligible when averaged over, for example, one day. In addition, when the automobile crosses a railroad using a d.c. electrical system at a time instance t.sub.o, it may be magnetized by a magnetic field produced by a d.c. current flowing through the rails and cables and thus the intensity and direction of the field Hv are considerably changed. With such change of the field Hv, the automobile must circle again to obtain the signals x.sub.v and y.sub.v corresponding to the changed field Hv. This is very difficult to do as a practical matter.